Recently, interests in energy storage technology have been gradually increasing. As the use of batteries is enlarged to applications for the storage of energy for portable telephones, camcorders, notebook computers, personal computers and electric vehicles, efforts on the research and development of batteries are increasingly embodied. In this view, the field of chargeable/dischargeable secondary batteries receives the greatest attention, and among them, interests in the development of lithium secondary batteries, which have a high energy density and a long cycle life, are focused upon.
In general, the lithium secondary batteries comprise a lithium metal oxide as a cathode active material, a carbonaceous material or a lithium metal alloy as an anode active material, and a solution containing a lithium salt dissolved in an organic solvent as an electrolyte. In the lithium metal oxide used as the cathode active material, the structural stability and capacity change according to intercalation and deintercalation of lithium ions. When a charge potential increases, the capacity increases, but a compound becomes structurally unstable. Such instability in an electrode structure may generate oxygen, thereby resulting in overheating within a battery, or causing explosion of the battery by a reaction with an electrolyte.
Organic solvents that have been used widely in recent years include ethylene carbonate, propylene carbonate, ethylmethyl carbonate, dimethoxyethane, gamma-butyrolactone (GBL), N,N-dimethyl formamide, tetrahydrofuran or acetonitrile. However, the organic solvents have enough volatility to cause evaporation, and are also highly ignitable, and thus are problematic in terms of stability under overcharge, overdischarge, short circuit and high temperature conditions, when applied to a lithium ion secondary battery.
More recently, many attempts including the use of a noninflammable ionic liquid as an electrolyte have been made mainly in Japan and USA in order to solve the above-mentioned problems. However, such a noninflammable ionic liquid has not been practically used yet because when the noninflammable ionic liquid is used together with the anode using a carbon based material and/or a lithium metal, the ionic liquid is reduced in advance of lithium ions at a higher voltage, or conductivity of lithium ions decreases due to high viscosity of the ionic liquid. Therefore, to overcome such disadvantages of the conventional organic electrolytes and ionic liquids, various attempts have been made to modify an electrode active material or develop a novel electrolyte containing an additive.